A plasma display panel includes a plurality of discharge cells arranged in the form of a matrix. In this plasma display panel, gas discharges are induced in the selected discharge cells, generating ultraviolet rays. The ultraviolet rays cause phosphors in the discharge cells to be excited, so that the discharge cells emit light. The number of times of discharges in the discharge cells per unit time, that is, the number of discharge sustain pulses applied to the discharge cells is controlled, thereby enabling display of luminance in multiple levels of gray.
Generally, a sub-field method is employed as a driving system of the plasma display panel. In the sub-field method, one field corresponding to one screen of video is temporally divided into a plurality of sub-fields, and the ratio of a light emission sustain period in each sub-field is set to a power of two, thus performing display in multiple levels of gray by combining the foregoing sub-fields. For example, when the ratios of the light emission sustain periods of eight sub-fields are set to 1:2:4:8:16:32:64:128, respectively, the sub-fields are combined to realize 256 levels of gray. Patent Document 1 discloses a technique relating to the sub-field method, for example.
Moreover, there exists a plasma display device having an ABL (Automatically Brightness Limit) function that variably sets the number of the discharge sustain pulses in each sub-field depending on an average peak level (APL) of an input image signal, mainly for purpose of reduction in power consumption. In the plasma display device having the ABL function, a characteristic curve indicating the relationship of the number of the discharge sustain pulses to the average peak level is stored in a memory, and the number of the discharge sustain pulses is determined depending on a detected value of the average peak level with reference to this characteristic curve.
In the above-described plasma display device, the ABL function causes the number of the discharge sustain pulses to be decreased in each sub-field when a high average peak level is detected, so that brightness of the whole screen is decreased. On the other hand, the number of the discharge sustain pulses is increased in each sub-field when a low average peak level is detected, so that brightness of the whole screen is increased.
For example, Patent Document 2 discloses a display control device having the ABL function for controlling a light emission luminance of the plasma display panel. The display control device holds in a storage a plurality of kinds of characteristic curves, which are a characteristic curve for standard use, a characteristic curve for preventing image burn-in, a characteristic curve for power saving and so on. A user can select any from these characteristic curves depending on the situation.
Here, description is made of the ABL function. FIG. 16 is a diagram showing the characteristics of the ABL function. FIG. 17 is a schematic view for use in explaining the ABL function.
FIG. 16 (a) shows the relationship between the average peak level APL and power consumption, and FIG. 16 (b) shows the relationship between the average peak level APL and the number of the discharge sustain pulses. The average peak level APL represents an average luminance level of video displayed by a video signal to be input. The larger the number of the discharge sustain pulses is, the higher the luminance level of the displayed video is.
As shown in FIG. 16 (b), the number of the discharge sustain pulses is kept constant irrespective of the average peak level APL in a region where the average peak level APL is lower than a certain value. In this case, power consumption rises with the increase in the average peak level APL as shown in FIG. 16 (a).
As shown in FIG. 16 (b), the number of the discharge sustain pulses is decreased with the increase in the average peak level APL in an ABL operating region where the average peak level APL is not less than the above-mentioned certain value. This causes power consumption to be kept constant irrespective of the average peak level APL as shown in FIG. 16 (a).
Consideration is made on a case where input video V1 having a comparatively low average peak level APL1 is supplied and a case where input video V2 having a comparatively high average peak level APL2 is supplied, for example. The input video means video displayed by the video signal to be input.
In this case, the number of the discharge sustain pulses when the input video V2 having the average peak level APL2 is supplied is lower by Op than the number of the discharge sustain pulses when the input video V1 having the average peak level APL1 is supplied. Thus, the luminance level of displayed video when the input video V2 is supplied is lower than the luminance level of the input video V2. Here, the displayed video represents video actually displayed on the screen.
FIG. 17 (a) shows the input video V1 having the low average peak level APL1. A partial region A in the input video V1 has a higher luminance level than that of a peripheral region B. FIG. 17 (b) shows the input video V2 having the high average peak level APL2. The partial region A in the input video V2 has a higher luminance level than that of a peripheral region C. The luminance level of the partial region A is equal in the input video V1, the input video V2.
In this example, the luminance level of the partial region A of the displayed video when the input video V2 is supplied is lower than the luminance level of the partial region A of the displayed video when the input video V1 is supplied.
As described above, the luminance level of an arbitrary region of the displayed video changes depending on the average peak level of the input video even when the luminance level of the input video is equal.
The ABL function suppresses the rise in power consumption with the increase in the average peak level. However, since the ABL characteristics cause the luminance level of the displayed video to be immediately controlled every time the average peak level changes, it has a disadvantage in that unnatural changes in luminance are visually recognized.
Therefore, Patent Document 3 discloses a luminance level control device for preventing the changes in luminance of the displayed video from being visually recognized.
FIG. 18 is a block diagram showing the configuration of a conventional luminance level control device disclosed in Patent Document 3.
A scene change detecting circuit 31 detects a scene change based on the input video signal. An average peak level operating circuit 32 calculates the average peak level of the input video signal, and outputs an average peak level signal indicating the result.
When the scene change is detected by the scene change detecting circuit 31, a luminance level switching circuit 33 outputs the average peak level signal output from the average peak level operating circuit 32. On the other hand, when the scene change is not detected by the scene change detecting circuit 31, the luminance level switching circuit 33 does not output the average peak level signal that is obtained in the average peak level operating circuit 32, but continuously outputs the same average peak level signal that has been output.
An automatically luminance limiting circuit 34 outputs setting data for setting a luminance and a video contrast suitable for the content of video based on the average peak level signal output from the luminance level switching circuit 33.
A luminance level control circuit 35 receives the setting data output from the automatically luminance limiting circuit 34, and outputs automatically luminance limiting data for controlling the luminance and contrast. A PDP (Plasma Display Panel) drive controller 36 causes the video to be displayed on a plasma display panel based on the input video signal and the automatically luminance limiting data.
In this manner, the luminance level of the plasma display panel can be controlled only when the scene change is detected.
[Patent Document 1] JP 2004-4606 A
[Patent Document 2] JP 2003-29698 A
[Patent Document 3] JP 2000-330505 A